A conventional package-on-package (PoP) structure provides an integrated circuit die coupled to a memory device using a bottom package that contains the integrated circuit die and a top package that contains one or more memory die. FIG. 1 illustrates a prior art PoP structure 100. A packaged integrated circuit die 110 may be mounted on top of a bottom package 125 using a flip-chip (i.e., controlled collapse chip connection) configuration so that solder balls provide a die to bottom package coupling 115. The bottom package 125 includes a bottom package coupling 112 (e.g., solder balls, bumps, dots, etc.) to couple the bottom package 125 portion of the PoP structure to a printed circuit board or other system substrate. The bottom package 125 and the top package 105 each include a substrate 130 and 135, respectively that provides a stiff layer on which additional insulating and conductive layers may be applied. The substrates 130 and 135 are typically implemented using a fiber-based material to provide a rigid structure. The substrate 135 may be 2 or more layers to accommodate routing from the top package to bottom package coupling 120 to a packaged integrated circuit die 132 that is mounted on the substrate 135 using a flip-chip configuration so that solder balls coupled the packaged integrated circuit die 132 to the substrate 135.
The bottom package 125 includes pads 122 on the top of the bottom package 125 that are coupled to a top package 105 by a top package to bottom package coupling 120 (e.g., solder balls). The solder balls that provide the top package to bottom package coupling 120 need to be sized to meet a clearance requirement between the bottom package 125 and the top package 105. Specifically, the top package 105 should not contact the die 110. The solder balls should also be positioned far enough apart from each other so that electrical shorts do not occur between adjacent solder balls. Therefore, as the clearance requirement increases, the size of the solder balls increase and the pitch between the top package to bottom package coupling 120 also increases. As the pitch increases, the number of separate electrical connections provided by the top package to bottom package coupling 120 for a PoP structure decreases. Typically, due to the clearance requirement, two rows of solder balls are used to couple the bottom package 125 to the top package 105, as shown in FIG. 1. A typical size for one of the solder balls in the top package to bottom package coupling 120 is 200 microns in diameter.
As the size of the solder balls increases, the reliability of the package-on-package structure 100 decreases due to solider ball collapse. More recently, PoP structures have been constructed using copper (Cu) cored solder balls. Because the copper cored solder halls are not susceptible to collapse, the pitch of the solder balls may be reduced compared with using conventional solder balls. Reducing the pitch of the copper cored solder halls may allow for the inclusion of a third row of pads and copper cored solder balls, thereby increasing the number of connections between the top package 105 and the bottom package 125. However, as memory bandwidth requirements continue to increase, even more connections between the top package 105 and the bottom package 125 are needed. Thus, there is a need for addressing these issues and/or other issues associated with the prior art.